The present invention relates to a data processing apparatus for a chromatograph and, more particularly, to a method of correcting a base line.
According to Japanese Patent No. 1,385,425 (Japanese Patent Publication No. 61-54181), as a method of correcting a base line of non-separation peaks, there are:                (1) the N method, (2) the θ method, and (3) the method of merely connecting a base portion by a straight line (non-correction method).        
In the N method, the number of peaks (N) is designated, N peaks are collected as one peak, and either a base portion or a trough portion is connected by a straight line (FIG. 3(A)). The base portion is a portion that is judged as not a peak area by using a change amount of signal as an index.
In the θ method, an inclination is intentionally loosened when the inclination seems to be too steep in the N method (FIG. 3(B)).
The non-correction method is a most typical method and is effective when the base line is estimated to be like a straight line (FIG. 3C). In addition, there is also a case of correcting a base line by intentionally selecting a forward horizon method (FIG. 3D), a backward horizon method (FIG. 3E), a special processing method of a shoulder peak (FIG. 3F), or the like in accordance with a peak shape of each chromatogram.
Each of the above base line correcting methods has both merits and demerits and is selected depending on a particular case, since it is difficult to unconditionally determine the base line. Although some methods in which the base line is unconditionally determined have been proposed (Japanese Patent Application Laid-Open Sho 62-32360, Japanese Patent Application Laid-Open Sho 63-88443, Japanese Patent Application Laid-Open Hei 6-94696, and the like), those methods are not yet generally used.
In all of the above methods, the base line is corrected on the basis of the base or trough portion existing in the chromatogram.
An algorithm of searching the base or trough portion generally tends to be too sensitive to a local fluctuation in a signal. For example, a case where the trough portion is detected and a case where it is not detected occur depending on a magnitude of noise. A detection of a starting point and an ending point of a peak, that is, an end point detection of the base portion is also disturbed by noise. After all, such a correction of the base line is easily affected by noise and quite different base lines may be obtained due to a slight difference in signals.
By a similar reason; there is a case where the base line largely fluctuates when parameters such as sensitivity, slope, and the like, to detect the base portion or trough portion, are improper.
In the case where the base line is experientially estimated to be horizontal, if a horizontal straight line is applied to the base portion, a proper base line can be obtained. In the case where the base line may possibly not be horizontal, however, a method of obtaining the base line by continuing the base or trough portion with a straight line like a graph of polygonal line is not always proper.